As is known in the art, high bleed temperature—pressure regulating pneumatic valves are commonly used for many A/C or other heavy duty industrial applications. As A/C application example, environmental control systems (ECS) often comprise valves and wing/engine lip anti-ice valves (ATVs) and the pressure regulation function of these valves is usually performed by means of a pressure relief valve (PRV). The purpose of the PRV is to establish the desired pressure set in a reference chamber (this reference pressure will be thus sensed by a sleeve piston or other mobile elements able to limit the pressure downstream of the main pneumatic valve).
The simplest concept of PRV is constituted by a plunger that is pushed against its seat by a spring. The spring preload is adjusted to reach the desired pressure set-point and when the pressure inside the reference chamber (which is continuously feed by a control orifice) reaches the PRV set-point (i.e. the force on the plunger seat overcomes the spring preload), the plunger displaces, thereby venting the control orifice flow. In this way, the desired reference pressure is established.
It is therefore clear that such PRVs heavily rely on the correct functioning of the spring element. First of all, the spring geometry (mainly in terms of spring faces parallelism) has to be tightly controlled in order to minimize transverse force to the plunger (which in turn causing friction and thus hysteresis on the reference pressure value with respect to upstream bleed pressure variation). Second of all, the spring preload, as well as the spring stiffness should not vary over time in order to guarantee a constant pressure set-point. The control of the combination of these two requirements (i.e. load stability together with tight dimensional control) is particularly challenging considering the high temperature the PRV is exposed to (engine bleed up to 700° C., PRV spring temperature up to 500° C.). Considering these temperatures, PRV springs are currently typically manufactured from Inconel® X750 or other suitable materials.
There is therefore a need to find an improved method of manufacture of these springs, and indeed to provide an improved method of thermally stabilising a material that may be used in this way.